Porous ceramic filters are used in a variety of applications, particularly where high resistance to chemical attack and/or thermal damage are required. One application of particular interest involves ceramic wall flow filters useful for the treatment of diesel engine exhaust gases. Such filters, termed diesel particulate filters (DPFs), are becoming increasingly important to meet more stringent exhaust emissions regulations limiting the amount of exhaust particulates (soot) that may be emitted by diesel passenger cars and trucks.
One very promising candidate for use in wall flow DPFs is cordierite, a magnesium aluminosilicate material exhibiting good strength and refractoriness, and a sufficiently low coefficient of thermal expansion to demonstrate good resistance to thermal shock damage. However, although many different formulations for porous cordierite ceramics have been developed, obtaining a proper combination of high strength, low thermal expansion, and a controlled pore morphology imparting both good gas permeability and high filtration efficiency remains difficult. Proper porosity is particularly important since low permeability leading to a high filter pressure drop is objectionable from the point of view of engine fuel efficiency and power output. Low pressure drops for both clean filters and for filters carrying significant loadings of trapped soot particulates are required.
A number of strategies have been developed to address these concerns. Published PCT application WO 2005/018790, for example, discloses active filter pretreatments comprising the application of passivating coatings to porous filters prior to the application of active catalyst deposits thereon. The passivating coatings significantly improved the properties of catalyzed DPFs, including filters composed of aluminum titanate as well as cordierite. Certain polymers pre-coated on porous ceramic surfaces were found to undergo ion exchange reactions with catalyst washcoat slurries, improving washcoat homogeneity on the porous surfaces of the filters while preventing washcoat diffusion into microcracks and fine pores in of the filters. Washcoat intrusion had been found to contribute to elastic modulus and thermal expansion increases in the filters and/or to degrade the permeability thereof.
Further improvements in filter performance were achieved in accordance with published PCT application WO 2005/091821, wherein certain thermally cross-linkable polymer solutions, when caused to gel within the filter pore structure, were found to promote the selective catalyst coating of larger pores in preference to smaller pores and microcracks within the ceramic. In consequence, both the catalyst efficiency and the gas permeability of these filters were enhanced.
Notwithstanding these improvements in catalyst coating technology, further improvements in the underlying porous ceramic structure will be needed if further improvements in catalytic wall-flow diesel particulate filter performance are to be realized. Filter performance is dependent on three fundamental properties, i.e., mechanical properties, thermal properties and filtration properties. All of these are dominated by the underlying ceramic structure, in particular the porosity, average pore size, pore size distribution, and pore interconnections that fundamentally affect filter permeability and filter pressure drop in both clean and soot-loaded configurations. While known filters can provide low clean pressure drops, low soot-loaded pressure drops, and low coefficients of thermal expansion, there remains a need in the art for a filter and method of making it wherein both reduced pressure drops and low thermal expansion coefficients would be secured.